Payload-carrying motor vehicles such as dump trucks are limited in their payload-carrying ability by various factors including the load capacity of their supporting axles and their related components including suspension and tires. Another limiting factor but which is quite distinct from vehicle component load limitations is government regulations. And all these factors impact on the economical use of a payload-carrying motor vehicle in that the more payload the vehicle can carry, the more profitable the vehicle can be.
With regard to the load limitations imposed by the axles and their functionally related components, a common approach to increasing the payload capacity has been to add one or more load supporting axles in the form of auxiliary axles commonly called pusher axles that are located between the steer axle and the drive axles; there commonly being two such drive axles employed in the heavy-duty vehicles. And in the case of pusher axles, it is common practice to suspend them in a retractable manner so that they may be deployed under the command of the vehicle operator as needed to aid in supporting the vehicle.
With regard to government regulations, the Federal Government in respect to bridges in the interstate system establishes weight limits on vehicles using the bridges wherein these limits are based on both the vehicle weight and the vehicle's wheelbase wherein the latter is measured as the distance between the center of the vehicle's foremost wheels and the center of the vehicle's rearmost wheels. With the greater this wheelbase, the more weight that is allowed under such restrictions.
As a result, a common approach to meeting such restrictions is to add a load bearing auxiliary axle commonly called a tag axle or trailing axle that is located a remote distance behind the vehicle and operates to significantly increase the payload capacity as well as the effective wheelbase of the vehicle. Examples of such in the case of a dump truck are disclosed in U.S. Pat. Nos. 5,823,629; 6,116,698; 6,247,712 and 6,189,901. Wherein this auxiliary axle, that will hereinafter be referred to as a tag axle, is suspended from the rear end of the truck's dump body and thereby the truck's frame and on vehicle operator command is conditioned in either a stored inactive condition or an active condition. And wherein in establishing the inactive condition, the tag axle is raised to a stored location above the rear end of the dump body out of the way of the dump body's tail gate and in establishing the active condition the tag axle is lowered to a remote location behind the truck frame and a down force is applied to the tag axle forcing the tag axle tires to bear against a road surface to thereby aid in supporting the truck frame as well as extending the effective wheelbase of the truck to a significant degree. And wherein the tag axle adds to the increased load capacity provided by other auxiliary axles commonly referred to as pusher axles that are located between the primary axles of the truck.
But in providing such a tag axle, it has been found that there are certain considerations to address that can enhance the ability of the tag axle and its functionally related components to perform to best advantage for the purposes intended. Such as the ability of the tag axle to efficiently support lateral loading on the vehicle frame tending to tilt the latter, efficiently accommodate certain varying road conditions at the tag axle wheels while addressing the effects of maintaining forced loading on the tag axle under certain road and tag axle suspension and tag axle tire conditions, efficiently accommodate braking the tag axle wheels while addressing the effect of such braking under certain road and tag axle suspension and tag axle tire conditions, and efficiently maintaining both forced loading on the tag axle and tag axle braking while addressing the effects of such under certain road and tag axle suspension and tag axle tire conditions.